Ambient ionization refers to a class of ionization techniques in which ions are formed in an ion source generally under ambient pressure conditions, and specifically outside of vacuum environment of a mass spectrometer (MS). Ambient ionization often, but not always, requires minimal or no sample preparation or analytical separation. Once analyte ions are formed they are transferred into the MS. Known methods for ambient sampling and ionization for mass spectrometry include direct analysis in real time (DART), electrospray ionization (ESI), and its many variants like desorption electrospray ionization (DESI), laser ablation ESI (LAESI), and matrix-assisted laser desorption and ionization (MALDI).
The DART technique typically entails generating a glow discharge plasma and flowing the plasma into contact with a sample whereby the plasma (primarily the metastable species) interacts with the sample. Depending on the feed gas used and the nature of the compounds under investigation, the DART technique can involve a highly complex set of ionization and fragmentation mechanisms. Penning ionization, impact ionization, and numerous chemical interactions occur between species that originate both in the discharge region as well as from the ambient air and the sample itself. In the absence of chromatographic separation, these reactions can result in highly complex and difficult-to-interpret mass spectra. Sample desorption in DART is convolved with plasma excitation and ionization. The DART technique therefore does not represent a practical method of analysis of unknown compounds. The DART technique typically requires substantial flow rates of discharge gas, typically in the range of 1 SLM (standard liter per minute) of helium. This level of gas consumption may not be economical for various applications, and would typically require large gas cylinders for frequent or continuous operation.
ESI techniques require the use of a solvent and a high-voltage needle. In ESI when implemented as an ambient ionization technique, particularly DESI, an electrically charged spray (electrospray) is produced when a high voltage is applied to a solvent, and the electrospray solvent droplets are attracted to a sample surface. Analytes from the sample are originally desorbed and then ionized by the charged aerosol. In laser-based ESI techniques, a pulsed laser incident on the sample surface ablates analytes from the surface, creating a plume of analytes above the surface and these analytes then interact with electrospray charged droplets to form ions. The ions produced by ESI-type techniques are introduced into an MS system via a vacuum interface.
MALDI requires the preparation of a sample in a matrix that acts as a laser absorber that facilitates laser ablation of a sample. The ablated and ionized sample molecules are then introduced into an MS system via a vacuum interface. MALDI is not appropriate for low molecular weight compounds (less than approximately 300 amu) due to interference from matrix peaks in the mass spectrum. That is, the matrix compounds typically employed in MALDI have molecular weights also less than 300 amu. Additionally the laser hardware used for MALDI is costly.
There is an ongoing need for devices, systems, and methods for performing analyte desorption and ionization while ameliorating or avoiding disadvantages associated with known ambient ionization techniques such as those noted above. There is also a need for such devices, systems, and methods that enable independent control over the respective desorption and ionization actions occurring during a sample run. There is also a need for such devices, systems, and methods that require little or no sample preparation or pre-treatment, or the use of solvents, or the use of costly high-precision instruments such as electrospray devices and lasers. There is also a need for such devices, systems, and methods that are capable of utilizing plasma for desorption, ionization, or both desorption and ionization. There is also a need for such devices, systems, and methods that are capable of generating plasma from a variety of different gas species instead of being limited to one or two species. There is also a need for such devices, systems, and methods that may be readily interfaced not only with mass spectrometers, but also with other ion-based spectrometers such as ion mobility spectrometers and further with optical-based spectrometers such as optical emission spectrometers.